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77:1 (2015) 205–218 | www.jurnalteknologi.utm.my | eISSN 2180–3722 |
Jurnal
Teknologi
Full Paper
A REVIEW OF THE CORROSION BEHAVIOR OF METALLIC
HERITAGE STRUCTURES AND ARTIFACTS
K. M. Daraina,b, M. Z. Jumaatb, S. M. Nazimuddina, A. Ahsanc*, R.
Rashidc, M. M. A. Azizd, M. Obaydullahb, A. B. M. S. Islamb
aArchitecture Discipline, Science, Engineering and Technology
School; Khulna University, 9208 Khulna, Bangladesh bDepartment of Civil Engineering, Faculty of Engineering,
University of Malaya, 50603 Kuala Lumpur, Malaysia cDepartment of Civil Engineering, & Institute of Advanced
Technology, University Putra Malaysia, 43400 Serdang, Malaysia dFaculty of Civil Engineering, & UTM Construction Research
Centre, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru,
Johor, Malaysia
Article history
Received
13 March 2015
Received in revised form
9 May 2015
Accepted
1 October 2015
*Corresponding author
ashikcivil@yahoo.com
Graphical abstract
Zinc corrosion in sculpture
Abstract
Awareness about restoring and preserving historically important structures and artifacts is
gradually growing in many parts of the world. These artifacts and structures represent the
culture, tradition and past of a nation. They are often also a source of national income
through tourist activities. Besides masonry and wood work, metallic forms and relics are a
vital part of the heritage which needs to be conserved. Certain metals have been used
significantly throughout history in the creation of objects and structures. However, metals
are prone to decay over time, particularly decay through corrosion. The basic mechanisms
of metal corrosion, the various types of corrosion and existing remedial solutions are
reviewed in this paper. The most significant factor affecting metal corrosion was found to
be the surrounding environment, especially in marine areas. Different remedial measures
can be implemented on corroded metals according to their specific properties.
Recommendations for further study are offered at the end of the paper.
Keywords: Corrosion, heritage structure, metal; artifacts, decay
Abstrak
Kesedaran mengenai mengelokkan dan mengekalkan bangunan bersejarah dan artifak
sedang berkembang di seluruh dunia. Artifak dan bangunan ini melambangkan budaya,
tradisi dan sejarah sesebuah negara Ia juga sering mengumbang kepada sumber
pendapatan negara melalui aktiviti pelancongan. Selain batu dan kayu, besi juga telah
digunakan dengan banyak di sepanjang sejarah penciptaan objek dan struktur. Namun,
besi sering mengalami pereputan semakin masa berjalan, terutamanya pereputan
melalui karat. Mekanisma asas bagi karat besi, jenis-jenis karat dan cara pemulihan sedia
ada telah dikaji dalam kertas kerja ini. Faktor paling penting bagi karat besi didapati
adalah keadaan persekitaran, terutamanya di kawasan marin. Pelbagai jenis teknik
pemulihan boleh dijalankan ke atas besi berkarat mengikut sifat-sifat tertentu besi
tersebut. Cadangan bagi kajian lanjut ada diberikan di penghujung kertas kerja ini.
Kata kunci: Kakisan, struktur warisan, logam, artifak, pereputan
© 2015 Penerbit UTM Press. All rights reserved
206 A. Ahsan et al. / Jurnal Teknologi (Sciences & Engineering) 77:1 (2015) 205–218
1.0 INTRODUCTION
Historic structures and artifacts are icons of the culture
and customs of a nation. The heritage of a nation is
composed of tangible and intangible components
such as architecture, art, history, archaeology, as well
as financial, social, political and religious or symbolic
practices and beliefs. Intangible heritage in the form of
professional craft knowledge is materialized in historic
objects as part of their intrinsic and sometimes hidden
and indiscernible value [1, 2]. According to B. M. Feilden
“a historic building is one that gives us a sense of
wonder and makes us want to know more about the
people and the culture that produced it”. He stresses
the emotional value and symbolism of cultural identity
and continuity, and qualifies a building as “historic”.
Interest in conserving historic structures and artifacts is
steadily increasing in many parts of the world [3].
Conservation is the action taken to prevent decay and
manage changes dynamically. It embraces all acts
that prolong the life of cultural and natural heritage. The
actions taken should be reversible and not prejudice
possible future interventions. The scope of conservation
in built environments is ranged from territorial planning
to the preservation or consolidation of a crumbling
artifact.
Tangible and intangible heritage have become a
source of income generation for many countries by
attracting people from other nations. According to
House of Commons (2004), heritage structures can act
as income generation sources by attracting tourists
from around the globe. Most studies in this field have
concentrated on masonry structures, although a
significant number of metallic structures have also
received attention [4-6]. In different civilizations metals
were used for different purposes, such as for structures,
artifacts and weapons [7]. As these metallic heritages
have aged, they have deteriorated and become
corroded. Primary pollutants such as SO2, NOx, and CO2,
under the influence of O2, humidity, sunlight, and
temperature react to form new secondary pollutants [5,
8].
Figure 1 Refining-corrosion cycle
The deterioration of metallic structures is mainly due to
corrosion, a natural process which has existed since
man first began to make and use metals [6, 9, 10]. This
process is a chemical reaction between metal and
other substances. The most common reagents are
oxygen and water. A layer of oxide forms on the surface
in this reaction. In some metals, if this layer is dense and
hard, it can be protective [11].
Corrosion is also affected by the position of metals in
the electrochemical series: Au > Ag > Cu > Sb > Sn > Pb
> Ni > Co > Cr > Fe > Zn > Mn > Al – Mg. Resistance to
corrosion is determined by the position of the metal in
the electrochemical series, in which metals are
arranged by their electrical potential. Any metal in the
above series is electronegative to all the metals which
precede it and electro-positive to all that come after it.
The process of corrosion is actually part of the cyclic
nature of extractive metallurgy. For example, as shown
in Figure 1, iron is manufactured from hematite by
heating it with carbon but gradually oxidizes and
degenerates into rust as it follows its natural life cycle
[12, 13]. Hematite and rust are similar in composition.
Historic places and structures are symbols of a
country’s culture and memorials of its past and deserve
to be preserved in their own right. However, for most
historic places and structures the main source of
income is from tourists. In order to attract tourists, these
heritage sites have to be protected and preserved from
damage and destruction. This paper presents an
extensive review of the corrosion process of historic
metal structures and artifacts. The aim of this paper is to
present a basic understanding of the corrosion process
of heritage metallic forms, which may be helpful to
engineers, stakeholders and restoration workers. The first
segment of this paper discusses the basic mechanisms
of metallic corrosion and the subsequent part discusses
remedial techniques and their procedural
implementation.
2.0 HISTORICAL METAL USE
Metals have been used by man for thousands of years.
The earliest communities and civilizations mainly used
metals to make weapons and tools. They explored the
use of metals and how to enhance the efficiency of
metals to prepare quality weapons.
Copper and one of its frequently used alloys,
bronze (90% Cu, 10% Sn), have been used since
ancient civilization. From historical evidence, it has
been found that copper and bronze have been in
use from at least 7000 BC.
Iron, in different forms such as wrought iron, cast
iron and steel, have been used since 4000 BC.
Zinc was first used during the 18th century.
Besides copper, iron and zinc, other metals such as
lead (Pb) and various alloys like brass (60% Cu, 40% Zn)
and nickel (65% Ni, 30% Cu) were used for different
purposes [14]. Tin (Sn) was occasionally used for
sculptures with Zinc (Zn) painted on the sculptures as a
protective coating.
Steel,
Fe+C(Si, etc.)
Fe
Fe2O3
(Hematite)
Iron Ore
[Fe2+]
Alloying
process
Energy
added(heat)
Corrosion
activity
Energy
liberated
Steel,
Fe+C(Si, etc.)
Fe
Fe2O3
(Hematite)
Iron Ore
[Fe2+]
Alloying
process
Energy
added(heat)
Corrosion
activity
Energy
liberated
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3.0 CONSIDERATIONS FOR HISTORICAL METAL
RESEARCH
Table 1 outlines several points that must be assessed
before proceeding with any research or repairing of
metallic heritage structures or objects.
Table 1 Parameters to evaluate usage and deterioration of
heritage metal
Research Parameters Items to consider
Understand
the
structure or
object
How does the object
or structure work
Construction
details
Effect resulting from
function (e.g.
fountain)
Integrity
Visible stress
fractures
External influences
Drainage,
subsidence, impact
damage, stress
fractures
Fixings Identify fixing
materials
Condition
Structural integrity
Materials Identify materials
used
Cast iron/wrought
iron/mild steel
Other materials
Combinations of
materials
Corrosion Intensity
Range
Uniform, localized
Analysis of
corrosion products
Coatings Sample
Stratigraphy
Record sample
locations
Gilding
Previous
repairs
Identify
Assess -
Recording
Photography
Drawn survey
Report
-
4.0 ATTRIBUTES OF HISTORICAL METALS
Conventional usage of metals can be classified into
two categories: Ferrous (containing iron) and non-
ferrous (containing no appreciable portion of iron).
Generally, non-ferrous metal objects are more corrosion
resistant. The general properties of metals that need to
be taken into consideration are density, malleability,
elasticity, ultimate tensile strength (tenacity),
coefficient of linear expansion, thermal conductivity
and specific heat.
4.1 Iron
Iron is a dark gray metal. The pure form of iron has been
found in meteorites that have crashed into earth from
outer space. Volcanic eruptions over millions of years
have deposited iron in the form of iron ore (rocks with
varying percentages of iron). Different civilizations have
attempted to extract iron from ore with different
success rates. Historically, the Romans improved iron
extraction technology and produced better armor. The
advent of the industrial revolution led to a better
understanding of the iron manufacturing process and
iron become more widely used [15, 16]. Different forms
of iron have been used over time. They are, in order of
appearance, wrought iron, cast iron, mild steel and
recently stainless steel. Figure 2 shows various exposed
structural steel samples with different levels of corrosion
severity.
Carbon steel wrought iron carbon steel
Figure 2 The unequal corrosion of different structural steels
4.1.1 Wrought Iron
Early forms of wrought iron were also called ‘charcoal
iron’. Wrought iron was generally manufactured
through a method called ‘direct reduction’. Iron ore
(iron oxide with various impurities) was smelted by
heating it with charcoal in small furnaces called
'bloomeries'. In this chemical process, carbon monoxide
is released as the charcoal diminishes the iron oxide to
iron.
Fe2O3 + 3C → 2Fe + 3CO (1)
Wrought iron is considered pure iron with a small
amount of carbon (less than 0.15%) and usually a little
slag. The mechanical strength of wrought iron is low.
Improvements in ductility and tensile strength can be
made by re-heating and re-working. It is soft and
malleable, but can be toughened and made fatigue
resistant. It has some resistance to corrosion. Up until the
14th century, wrought iron was mainly used for spears,
swords and knives. Structurally, its use was restricted to
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tie bars. Starting from the 18th century, this metal was
also used for beams and girders.
(a)
(b)
Figure 3 Examples of wrought iron [17]
Wrought iron objects were used in ancient civilization
[18]. The Romans commonly used wrought iron doors. Its
structural use dates from the Middle Ages when
wrought iron bars were occasionally employed as tie
bars for masonry arches and domes. Due to its superior
tensile capacity, usage of wrought iron dominated the
canal and railway ages, as cast iron is strong only in
compression [9, 11]. A fractured wrought iron bar in
Figure 3 shows the fibrous structure which makes
wrought iron ideal for working under the hammer [17].
4.1.2 Cast Iron
Cast iron is considered an alloy. Its high carbon content
(1.7% to 3.7%) gives it a better corrosion resistance
capacity than wrought iron or steel [12, 19]. Variable
amounts of silicon, sulfur, manganese, and phosphorus
are also present in this alloy. Different techniques and
compositions used during melting, casting and heat
treating influence the characteristics of the final cast
iron product.
Figure 4 Cast iron steps with highly decorative patterns
Various decorative and structural forms can be
produced from molten cast iron [20]. The properties of
cast iron are quite different from steel and wrought iron.
Cast iron is too hard and fragile to be shaped by
hammering, rolling, or pressing. It possesses greater
compressive capacity, though weak in carrying tensile
loads. Unlike the other forms of iron, it can withstand
buckling loads with more rigidity. However, its inferior
tensile capacity is disadvantageous as there is no
warning before failure.
As cast iron is able to carry high compressive loads,
mainly compression members like columns, struts and
staircases were made from it (Figure 4). Cast iron often
contains mold lines, flashing, casting flaws, and air
holes. However, cast iron elements are very uniform in
appearance and are frequently used repetitively. Cast
iron elements are often bolted or screwed together
[21].
Table 2 The effect of corrosive agents on bronze
Agent Salt formed Color of patina
Oxygen Oxide Red brown to dark
black
Oxygen
and
chlorine
Chloride/Oxychloride Very pale green
Carbonic
acid Carbonate Green
Sulphuric
acid
Sulphate/Basic
Sulphate Deep blue/Green
Nitric acid Nitrate Blue green to blue
Sulphur Sulphide Dark brown or black
Cast iron is vulnerable to a specific kind of corrosion,
called graphitization, where porous graphite is
impregnated with insoluble products. Graphite and iron
silicon act as electrodes and dissolve the iron matrix.
The cast iron objects preserves their appearance and
profile, but are structurally weakened. This
phenomenon generally happens when cast iron
products are not painted for a long duration, when
joints fail and acidic rainwater corrodes pieces from the
exposed side.
4.2 Copper
Copper is widely used for architectural purposes as it
has a high corrosion resistance capacity. The
commonly used copper alloys are:
Brass – Copper is alloyed with zinc (Zn) with
copper varying from 90-60%, which affects
properties and color of the metal. Sometimes
magnesium and aluminum are also added.
Muntz metal – Around 60% copper and 40%
Zinc is used to make Muntz metal. This alloy is
209 A. Ahsan et al. / Jurnal Teknologi (Sciences & Engineering) 77:1 (2015) 205–218
generally used for decorative tiles, sheets and
plainer surfaces.
Copper develops a patina by weathering. Especially
in roofing the patina can vary depending on exposure.
Usually, it will first become a coppery red, gradually
deepening to a dark brown. In lighter corrosive
environments, a green patina may develop in the form
of copper carbonate. In highly polluted environments,
the patina may change from blue to black. Severe
corrosion attacks of copper roofs are caused by
acidified rainwater runoff from other surfaces or by
concentrated flue gases.
4.3 Bronze
Bronze is an alloy of 90% copper and 10% tin. Silver may
be present in small quantities. Generally, its color is a
salmon gold. However, this is seldom seen without the
dark brown-red or green patina of the oxidized surface
in corrosive urban or industrial atmospheres. Table 2
shows the effects of corrosive agents and the color of
the patina they form on bronze components. Bronze
has been used for casting sculptures, cannons, bells
and other architectural elements such as doors since
prehistoric times. Bronze reacts with alkalis such as
ammonia and with various sulphurous compounds that
combine with water to form sulphuric acid. Direct
contact of copper with iron, steel, zinc, aluminium or
galvanized steel may cause these metals to corrode.
In some situations the patina can be considered
aesthetically valuable. Therefore, before any
intervention measures are taken to remove the patina
from the surface, the bronze object or structure should
be carefully assessed.
Figure 5 Environmental factors that affect metal corrosion [22]
5.0 BASIC CAUSES OF METAL CORROSION
Climatic parameters play a significant role in the
corrosion of metals. Figure 5 shows that the
temperature, humidity and rain have a significant
influence on the condition of metals. When metals
come in contact with gaseous pollutants and other
reactive environmental agents, they chemically react
to form different kinds of precipitates and slowly
become corroded.
Sometimes allowances for corrosion are given in the
design of a metallic object or structure by considering
the possible duration of service life, the thickness of the
material and other service conditions. Table 3 presents
the influence of various conditions including humidity,
temperature, rain, wind, impurities and metal wet times
in different areas on corrosion rate [23]. Temperatures
above 00C and relative humidity over 80% foster
corrosion. Dissolved air impurities and adherent dust or
dirt also facilitate atmospheric corrosion. The table
below shows the uniform steel corrosion rate in different
atmospheric conditions.
Table 3 Steel corrosion rates in various atmospheric conditions
Atmosphere Corrosion rate
(μm/year)
Rural 4-60
Urban 30-70
Industrial 40-160
Marine 60-170
A layer of electrolyte on metal surfaces is one of the
basic conditions for corrosion. Electrochemical
reactions, anywhere from 5-150 µm, occur in this layer
of electrolyte. If any amalgamation was done with two
metals and if there is a fair difference between their
positions in the electrochemical series, then corrosion
will occur at the anode part [24].
Galvanic corrosion causes the oxidation of the anode
metal and discharges electrons to the cathode metal,
which remains undamaged (Figure 6) [25]. The
following conditions must be present for the reaction to
occur:
A variance in electrochemical potential
between two neighboring metals.
The surface of the metal object should be
covered in an electrolyte.
An electrical track should be present to
facilitate electron movement.
(a)
210 A. Ahsan et al. / Jurnal Teknologi (Sciences & Engineering) 77:1 (2015) 205–218
(b)
Figure 6 Schematic representation of galvanic influence on
corrosion [10]
The water is disassociated into hydrogen ions and
hydroxyl ions:
H2O = H+ + OH¯ (2)
Surface ionization takes place when a metal such as
iron is posited in a liquid:
Fe = Fe++ + 2e¯ (3)
Ferrous ions moving away from the metal surface are
further oxidized to ferric ions:
Fe++ = Fe+++ + e¯ (4)
These Fe+++ ions are attracted to the (OH) ¯ ions and
form the corrosion product Fe (OH) 3.
Fe+++ + 3(OH)¯ = Fe(OH)3 (5)
Figure 7 Pourbaix diagram for iron in water at 250C [28]
5.1 Pourbaix Diagram
Marcel Pourbaix devised this diagram to represent the
effect of pH on equilibrium potential. The diagram is
generally used to illustrate corrosion possibilities [26-28].
The diagram in Figure 7 visualizes the states of iron when
potential and pH are varied. When the potential is
below the lines a and b, the metallic state is stable and
the metal is immune to corrosion [29].
As can be seen from the diagram, the pH–potential
plane is clearly divided into a corrosion region, a
passivity region and an immunity region (Figure 7). There
is also a small corrosion region at high pH, where the
dissolved corrosion product is HFeO2¯. The diagram can
be used to determine the possible ranges of
environmental pH and potential at which corrosion can
be avoided.
5.2 Corrosion Under Oxygen Reduction
The presence of hydrogen and oxygen is essential for
electrochemical reactions to take place [30]. Excessive
amounts of H+ ions in sufficient amounts for reactions to
occur are only found in acidic environments [31].
Oxygen reduction is possible in neutral and alkaline
mediums as electrochemical reactions are supported
by the continuous dissociation of water.
4H2O → 4H+ + 4OH¯ (6)
O2 + 4H+ + 4e¯ → 2H2O (7)
Total reaction: O2 + 2 H2O +4e¯→ 4OH¯ (8)
The oxygen reduction in acidic liquids is described by
equation (7) and in neutral and alkaline solutions by
equation (8). The reversible potential of the two
reactions as a function of pH is represented by the same
straight line in the Pourbaix diagram.
At a given pH value and 25°C the reversible potential is
E0 = – (9)
= – (10)
Figure 8 Concentration of oxygen in air-saturated water as a
function of temperature [31]
211 A. Ahsan et al. / Jurnal Teknologi (Sciences & Engineering) 77:1 (2015) 205–218
Figure 9 Corrosion of steel in water as a function of
temperature [31]
5.3 Effect of Temperature
Temperature affects the exchange current density and
the Tafel gradient. In natural environments, the most
significant effect of temperature is often its effect on the
diffusion-limiting current density iL [31, 32]. On surfaces
without diffusion-limiting deposits, iL is expressed by:
(11)
The diffusion coefficient, , depends strongly on the
temperature:
(12)
Where A and Q can be considered as constants in
water, R the universal gas constant and T the
temperature in K. The thickness δ of the diffusion
boundary layer depends on different factors. For open
systems where water is in equilibrium with the
atmosphere, the oxygen concentration cB is
determined by the solubility, i.e. the saturation
concentration, of air in the water. This concentration
depends on the temperature, as shown in Figure 8. The
effects of D and cB together lead to the corrosion rate
as a function of temperature as shown in Figure 9, curve
a) for an open steel tank, with a marked maximum at
about 80°C [31].
The thickness of the diffusion boundary layer depends
on the flow velocity for a plane metal surface, which is
shown in equations (10) and (12) [31]. The conventional
Nernst’s diffusion boundary layer (Figure 10) can be
expressed by:
(13)
Where
Re is Reynolds’ number = vL/Q,
Sc is Schmid’s number = ν/D,
v is the free flow velocity
L is a characteristic length and f(L) is a simple
geometrical function
Figure 10 Nernst’s diffusion boundary layer [33]
Figure 11 Corrosion pattern of archeological iron objects
covered in earth [36]
5.4 Estimation of Average Corrosion Rate
The estimation of average corrosion rates (CR) is of
great importance to predict the long-term corrosion of
metals [16, 34-36]. Using this method an archeological
iron artifact’s corrosion can be calculated and its
reliability can be verified by solubility calculation [37].
Figure 11 shows a diagrammatic representation of the
corrosion pattern in buried archeological iron objects.
The corrosion rate can be obtained by dividing the
iron equivalent thickness (teq) of the corrosion products
by the age of the sample:
(14)
Iron equivalent thickness is calculated from the
thickness of the corrosion products (dense product
layer and transformed medium) corrected by the ratio
of the density of the local medium and of iron as follows:
(15)
Where
t = distance from metal/corrosion products interface
dt = elemental variation of t
ρ(t) = density of medium (metal, corrosion products,
soil) at t
ρFe= density of iron (7.8 g·cm–3)
mass%Fe(t) = quantity of iron at t in mass%.
212 A. Ahsan et al. / Jurnal Teknologi (Sciences & Engineering) 77:1 (2015) 205–218
Another hypothesis, given by Vega E, Dillmann P,
Berger P and Fluzin P [38], is that the transportation of
oxygen in the water at the dense product layer pores is
the limiting step in the corrosion mechanism. The
corrosion rate can be evaluated using Faraday’s law
and taking into account the oxygen concentration
gradient (mol/m3), the thickness of the layer x0
(m), the iron density μFe (g/m3), the molecular mass M
(g/mol) and the apparent oxygen diffusion coefficient
(m2/s).
(16)
6.0 CATEGORIES OF CORROSION IN
HISTORICAL METAL
There are various types of corrosion in different metals
(Figure 12). Corrosion may start with one type of
corrosion, which can then lead to a series of worsening
reactions where other types of corrosion attacks can
become dominating factors [39]. The behavior and
causes of corrosion can be found through careful
investigation of the corroded sample while it is
corroding, which may include microscopic
examination of the surface of the metal object with the
corrosion products intact and also after the corrosion
products have been removed.
Figure 13 shows various types of corrosion that
frequently occur on metals. These forms of corrosion are
organized based on the simplicity of their detection.
Some of these corrosions can be detected simply by
visual inspection, such as uniform corrosion, whereas
others need extra testing and special equipment, such
as fatigue corrosion. Table 4 describes various
categories of corrosion, their basic reasons of formation,
significant attributes and possible remedial measures.
Uniform corrosion Pitting corrosion
Lamellar corrosion Erosion corrosion
Intergranular corrosion Dealloying corrosion
Crevice corrosion Galvanic corrosion
Figure 12 Various types of corrosion in different metals [31]
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Cavitation corrosion Fretting corrosion
Stress corrosion Fatigue corrosion
Figure 13 Major appearances of corrosions arranged by their
ease of identification [31]
Table 4 Basic mechanism, characteristics and possible
remedies of various corrosion types
Corrosion
category Causes
Important
features
Remedy/comme
nts
Uniform
corrosion
Lack of
substantial
passivation
tendency for
homogeneo
us materials
in actual
environment
[40].
The damage
propagates
quite evenly on
the entire
surface which
causes a
reduction in
thickness.
Conventional
protection
methods such as
coating,
cathodic
protection and
environmental or
material changes
are possible
remedial
solutions.
Pitting
Occurs in
passivated
metals when
electrode
potential
exceeds a
critical value
in an
environment
containing
chloride,
bromide and
iodide ions
[41, 42].
Narrow pits
form with
radiuses of the
same order of
magnitude.
The pits may
have different
shapes but
sharp
boundaries.
This is a serious
condition as
the pits may
penetrate
without clear
warning and
maximum pit
depth
increases with
increasing
surface area
[41].
Appropriate
materials that are
sufficiently active
against harsh
environments
provide the best
protection.
Cathodic
protection also
works.
Crevice
corrosion
A localized
mode of
corrosion
related to
stagnant or
even flowing
liquid at the
micro-
environment
al level that
tends to
occur in
crevices.
Most active
in presence
of chloride,
though there
is evidence
of activity in
other salt
solutions.
Influenced by
numerous
factors such as
metallurgy,
environment,
electrochemic
al, physical
surface,
geometrical
nature and
crevice gaps
[42, 43].
Newer high-alloy
steel with high Mo
can be used
instead of
susceptible
conventional
metals (e.g.
stainless steel).
Crevices and
possible
deposition should
be avoided (e.g.
butt weld instead
of overlap,
straight-forward
drainage paths).
Cathodic
protection is also
possible.
Pack rust
A form of
localized
corrosion in
crevices but
develops in
open
atmospheric
environments
.
Rust packing
appears
between two
steel plates,
especially in
steel bridges
[44].
Galvanic
corrosion
Metallic
contact
between a
more noble
and less
noble metal
through an
electrolyte
causes
corrosion to
increase on
the latter and
decrease on
the former
[45].
Corrosion
medium and
temperature
influence
galvanic
corrosion[46].
Aluminium and
aluminium
alloys are
particularly
prone to this
type of
corrosion [47].
The welded
metal should be
more noble than
the base
material.
Blending of a
large area of old
material with a
smaller area of
new, more active
material should
be avoided. The
two metals can
also be insulated
from each other
or non–metallic
distance pieces
can be used. A
metallic coating
on one of the
materials can
also work.
Erosion
corrosion
A relative
motion
between an
eroding
liquid and a
submerged
metallic
object can
cause the
exposed
material
surface to
mechanicall
y wear away
or erode [48].
Grooves or pits
form
according to
the direction
and condition
of the flow.
Usually occurs
at
comparatively
high velocities
between the
material
surface and
the fluid [49].
Properly
designing flow
systems and other
apparatus,
filtering or
precipitating solid
particles,
selecting suitable
materials,
applying
corrosion-
resistant coatings
and using
cathodic
protection are
possible
preventive
measures.
Cavitation
Similar to
erosion
corrosion but
the
appearance
of the
corrosion is
different. It
takes place
when flow
velocities are
excessive
and fluid
dynamics
cause large
pressure
variations
[50].
Deep pits
develop
perpendicularl
y to the
surface, often
localized close
to each other
or grown
together over
smaller or
larger areas,
making a
rough, spongy
surface [51].
Possible remedies
are using metals
high in hardness
and resistant to
corrosion,
increasing the
fluid pressure to
avoid gas
bubbles,
applying
corrosion-
resistant coatings,
cathodic
polarization and
designing to
avoid vibration.
Fretting
corrosion
A minor
repeated
relative
motion (slip)
at the edge
between two
May direct to
more serious
macroscopic
motion
between parts
or initiate
Can be
prevented by
using lubricants
(e.g. low-viscosity
oil, molybdenum
sulphide) or
214 A. Ahsan et al. / Jurnal Teknologi (Sciences & Engineering) 77:1 (2015) 205–218
tightly fitted
elements
[52].
fatigue crack
[53].
obstructing
oxygen intrusion
with gaskets or
sealants.
Intergranul
ar
corrosion
Galvanic
activity due
to variances
in impurity
concentratio
ns or alloying
component
materials.
Localized
damage on or
at grain
borderlines
with minor
corrosion at
other parts of
the surface.
Dangerous as
the toughness
of the
substance is
extremely
reduced at a
comparatively
early phase,
and a fracture
can take place
without notice
[54, 55].
Intergranular
corrosion occurs
in stainless steels
and alloys based
on nickel,
aluminium,
magnesium,
copper and cast
zinc.
Stress
Corrosion
Cracking
Cracks form
due to
concurrent
effects of
static tensile
stresses and
corrosion
[56]. The
tensile
stresses may
originate
from external
loads or
temperature
changes, or
they may be
internal
stresses
induced by
cold working
or heat
treatment
[39, 57].
If cracks are
not detected
on time, they
may cause fast
unstable
fracturing [16,
51].
Environmental,
electrochemic
al and
metallurgical
factors as well
as mechanical
stress and strain
are responsible
for the
mechanism
and
development.
Reducing the
stress and stress
intensity to below
threshold values,
making the
environment less
aggressive by
removal of
oxygen,
distillation or ion
exchange, using
cathodic
protection,
supplying
inhibitors and
selecting the right
materials are
measures that
can be taken to
relieve this type of
corrosion.
Corrosion
fatigue
Cracks are
stimulated by
fatigue
(variations in
stresses) and
are further
accelerated
by corrosion
[58]. Varying
tensile
stresses
causes this
deterioration
.
Fatigue
fractures in
non-corrosive
environments
exhibit large
smooth crack
surface areas
where the
cracks has
grown by
fatigue and an
(often smaller)
area with a
rough and
crystalline
surface formed
by fast
fracturing
when the
maximum
stress reached
the ultimate
strength [57,
59].
Measures to
prevent this type
of corrosion
include reducing
tensile stress levels
as much as
possible by stress
annealing,
applying reliable
coatings on the
highest fatigue
areas, changing
the environment
by using inhibitors
and de-aeration,
selecting
appropriate
materials and
proper designs,
using moderate
cathodic
polarization and
anodic
protection.
7.0 CLASSIFICATION OF ENVIRONMENTS FOR
CORROSION
Different environment have different levels of
corrosiveness (Table 5) [60]. There are several standards
that address the level and magnitude of corrosive
environments [61, 62]. One of the defining standards for
classification of corrosive environments is ISO 9223:1992,
which classifies the corrosiveness of different
atmospheres on various metals and alloys.
Environments are organized based on three key factors:
wet time, sulphur dioxide pollution and chloride content
in air [63]. Guiding values for the corrosion rate of steel,
zinc, copper and aluminium in the first three years are
categorized in ISO 9224. Two other standards also
categorize the corrosion of metals through the
atmosphere and immersion. EN ISO 12944-2 classifies
environments by their corrosive effects on steel
structures protected by paints and varnishes. ISO 14713
discusses the protection of iron and steel structures
against corrosion by using zinc and aluminum coatings.
Table 5 Corrosive effect of different environments on carbon
steel and zinc
Corrosivity Example
environments
Carbon
steel
(μm/year)
Zinc
(μm/year)
C1 (very
low)
Indoor: Spaces
with occasional
condensation
Outdoor: Inland
rural
≤1.3 ≤0.1
C2 (low) Indoor: Dry
spaces 1.3-25 0.1-0.7
C3
(medium)
Indoor: Spaces
with high
moisture
content, few
impurities
Outdoor: inland
urban, mildly
saline
25-50 0.7-2
C4 (high)
Indoor:
chemical
industries,
swimming pools,
seaside docks
Outdoor: very
humid industrial
plants, seaside
urban areas
50-80 2-4
C5-I (very
high)
Outdoor: very
humid industrial
atmospheres
80-200 4-8
C5-M (very
high)
Outdoor: saline
seaside
atmospheres
80-200 4-8
8.0 COMMON TESTS FOR DIAGNOSIS
When any metallic artifacts are discovered,
archeologists and scientists usually attempt to
determine the main features and flaws of the heritage
215 A. Ahsan et al. / Jurnal Teknologi (Sciences & Engineering) 77:1 (2015) 205–218
metal. This includes ascertaining the homogeneity of
casting, critically thin areas, the structural continuity of
joints indicating voids in the casting, previous repairs,
and micro cracks. Various methods including non-
destructive testing are employed to locate these
aspects. Some of these methods are X-ray diffraction,
ultrasonic testing, radiography thermo vision and
acoustic emission. Chemical and electrochemical tests
following different codes and standards are also used
to determine the extent of corrosion and other
mechanical and chemical properties of retrieved
metallic artifacts.
(a) Bronze corrosion in
sculpture
(b) Cast iron corrosion in
sculpture
(c) Lead corrosion in
sculpture
(d) Zinc corrosion in sculpture
Figure 14 Different forms of corrosion in historic artifacts [64]
9.0 CORROSION PROTECTION
Corrosion prevention usually involves the application of
a coating. Cathodic protection using a zinc coating or
galvanizing can also suppress corrosion. Widespread
surface protection approaches are:
Protective paint coating
Electroplating
Hot dip galvanizing
Coil coating of sheet steel
Rubberizing
Spray galvanizing
Chromium plating
Aluminum spraying
An organic topcoat is sometimes applied to roofing or
cladding products to enhance appearance and
increase durability. The thickness of this coating may
vary from 25-200μm. Paint is a barrier coating which
generally provides enough corrosion protection for
metals in many applications. However, paint containing
red lead is toxic, thus, zinc phosphate paint is a better
option. It is recommended that two coatings are
applied. Examples of protective coatings that can be
painted on are alkyd resin paints, chlorinated rubber
and vinyl solvent paints, epoxy or polyurethane
coatings. Zinc and aluminium coatings provide
cathodic protection to iron.
Cast iron and wrought iron are of intrinsic value and it
is thus preferable to repair as to renew objects and
structures made from these materials [18]. Figure 14
shows different forms of corrosion on various historic
artifacts. Repairing and preserving these historic
metallic structures and artifacts is a better choice than
removing them [13]. Historic fabrics and traditional
materials and techniques should be maintained all
through the consolidating process [64], which may
include the use of additional materials or structures to
reinforce, strengthen, prop, tie or support the existing
object or structure.
Welding is not always possible for all metals. Excessive
heat can cause recrystallization. It is inadmissible to
weld large sections of cast iron on site. Wrought iron can
be welded satisfactorily, and can be welded to steel
and stainless steel. Cold repair methods include straps,
threaded studs screwed into both sides of a fracture
and dowels or plain pins. Seriously corroded, broken or
missing castings may need to be recast. Gray cast iron
should be replaced in the same material.
Where copper has been perforated, copper patches
can be soft soldered over the hole. The patch should
be large enough and correctly shaped to cover the
area. The patch and holed sheet are seated together
with continuous pressure applied to the patch during
soldering. If the corrosion attack is in the early stages
only the cause of the attack needs to be eradicated
[65, 66].
Lead should never be repaired with solder as the
different thermal expansion rates will cause the solder
to break away. Lead in the edge zone tends to adopt
a crystalline structure which does not accept welding.
If deterioration is localized on the upper surface, the
damaged section can be removed and replaced.
Mastics or bituminous compounds cannot be used in
repair because they obscure the source of the
problem. The initial white patina of basic lead
carbonate can be washed off. The formation of patina
can be controlled by an application of patination oil.
Zinc sheet was once popular as roofing in France,
Germany and Belgium. Zinc is less malleable than lead.
Zinc roofing is initially bright, but with exposure to the
atmosphere gradually develops a dark gray patina of
zinc carbonate. The under surfaces can develop
condensation corrosion. Zinc resists contact with other
metals except copper and its alloys. It is prone to
corrosion from sulphur products, acids and strong
alkalis. The best way to clean zinc is with a brush and
water. Zinc roofing is best left unpainted.
10.0 ANTICORROSION PAINTS
Before coating a metallic structure or object with
protective paint, the surface to be painted must be
perfectly clean. Surface preparation and cleanliness
play a crucial role in the effectiveness of anticorrosion
216 A. Ahsan et al. / Jurnal Teknologi (Sciences & Engineering) 77:1 (2015) 205–218
paints. There are several standards for the pretreatment
before applying paints. The inspection procedure for
steel surface cleanliness is discussed in ISO 8501-1 and
ISO 8501-2. The standards for protective paint systems
are stated in ISO 12944-1 to 8.
The preparation of a sound surface before applying
protective paint involves the removal of old paint, rust,
loose mill scale and soluble corrosion salts. The analysis
of paint layers is part of the preparation program. Sound
paint surfaces may simply be rubbed down. Small areas
of paint can be removed with thixotropic paint strippers
such as methylene chloride and their residues removed
with white spirit. Flame cleaning and hot air blowers are
also effective. Flame treatment is preferred in cleaning
wrought iron as it removes only loose mill scale. In
cleaning iron surfaces, the rust must be completely
removed. Ferrous sulphate and chloride are water
soluble, though not readily removed. The surface should
be tested several times after each cleaning action.
11.0 CONCLUSIONS
Heritage structures and artifacts are a prime concern
for peoples and nations around the world. The tangible
and intangible elements of heritage are important as
they are symbols of the history, culture, customs and
glory of a nation. Cultural tourism is also now an
established income generation source for any heritage
enriched country. This has led to a pressing demand for
the conservation and restoration of these historic forms
and relics. In this paper an extensive survey of literature,
including current studies, has been carried out. The
main focus of this paper has been on the decay
mechanisms and its remedial measures for historic
metallic structures and objects.
Metals commonly used over the course of history
have been studied for their manufacturing process,
chemical composition, behavior and general usage. It
was found that corrosion is the most common cause of
decay in historic metallic structures and artifacts. The
basic reasons for corrosion related problem and their
guiding mechanisms were revealed in this study.
Several guiding parameters which have great influence
on the corrosion behavior of metallic structure were
discussed. Among the most important factor
influencing corrosion is the surrounding atmosphere. It
plays a very important role in metal corrosion. Different
atmospheric conditions cause the severity of the
corrosion to vary. A classification of different
atmospheres, their corrosiveness and their practical
limits were presented in this study. Marine environments
were found to be highly corrosive due to the presence
of salt ions in the atmosphere. This paper also made a
general classification of known corrosion problems, their
impact and possible remedial measures. It was found
that some types of corrosion (e.g. uniform, pitting,
galvanic corrosion, etc.) are more common in heritage
structures, whereas others appear rarely (e.g.
cavitation, lamellar, intergranular corrosion, etc.).
Conventional procedures such as coating and
cathodic protection were found to be effective
protection techniques. However, surface preparation
and cleaning in accordance with standard guidelines
are necessary before applying these techniques.
Some significant areas which need to be further
explored in future studies are:
More effective conservation techniques to
protect metallic heritage structures located in
marine environments need to be devised.
Climatic changes threaten many cultural
heritage structures around the world with
accelerated decay. This issue needs to
addressed globally and suitable measures
need to be taken.
The consequences of atmospheric pollution
(SO2, NOx and other suspended particles) are
ubiquitous. The levels of pollution need to be
properly monitored and the effects on heritage
structures and relics mitigated.
Compatible, feasible and sustainable materials
and intervention techniques which will
maintain the historic fabric of heritage
structures need to be investigated.
The true scale of environmental impact on
heritage structures within buildings, cities, sites,
etc. needs to be assessed and controlled.
A multidisciplinary knowledge based decision
making system for the sustainable preservation
of these heritage structures and artifacts needs
to be established.
Acknowledgement
The authors gratefully acknowledge the support given
by University of Malaya (UM) for funding the study
through the University of Malaya Research Grant
(UMRG) RP018-2012A.
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